One of the most efficient ways for generating ozone, O.sub.3, is to subject oxygen, O.sub.2, or a gas containing a high concentration of oxygen to a corona discharge. Thus, for example, ozone may be generated by passing oxygen through an annular gap between a glass tube and a grounded outer electrode and applying a cyclic high voltage to an inner electrode embedded in the glass tube. Ozone is typically generated during portions of cycles occurring just prior to a peak. Therefore, more ozone can be produced by increasing the frequency of this cyclic voltage, but a point is reached when the power dissipated in the annular gap tends to break the ozone molecules O.sub.3 down into oxygen molecules O.sub.2.
Circuits for generating the drive voltage for ozonators have used auto transformers or saturable reactors, both of which are bulky, heavy and inefficient. Furthermore, it is desirable to be able to control the rate of production of ozone from 0% to 100% of the capacity of the ozone generator by controlling the drive voltage. This is impracticable to achieve with circuits using saturable reactors. In addition, an ozonator presents a capacitive load to the drive voltage supply so that the sizes of the step-up transformer and inductive control components of the supply are much larger than would be required if the power factor were unity.
In a U.S. patent application entitled "Power Supply"
that was filed on Oct. 23, 1991 by the present inventors of this application, under Ser. No. 07/781,793, the drive voltage cycles supplied to the ozonator is asymmetric so that its slope during an increase in voltage to a maximum value was less than the slope during a decrease in voltage from the maximum value to zero. Since, as previously noted, ozone is produced while the drive voltage is increasing from some predetermined value to its maximum value, ozone is produced during a greater portion of each cycle than would be the case if the drive voltage half cycles were symmetrical. Control of the rate at which ozone is produced in this system is achieved by varying the duration of the lower slopes of the cycles. Operation at near unity power factor is achieved by including an inductor in the circuit that resonates with the capacitance of the ozonator at a frequency that is just below the frequency of the drive voltage so that the AC power supply uses an inductive load. The drive voltage cycles are generated by switching diagonal pairs of switches of a full bridge circuit.
A brief review of the teachings of prior references now follows. Invertor circuits for synthesizing an AC voltage waveform from a DC voltage are shown in U.S. Pat. Nos. 3,667,027; 4,002,921; 4,191,993; 4,191,994; and 4,922,401. Each one of these systems uses SCR or similar switching elements for developing the synthesized waveforms. Transformer coupling is used in a number of these known systems.
Huynh et al. U.S. Pat. No. 4,680,694 teaches a full-wave invertor using four thyristor switching elements T.sub.1 through T.sub.4. It is indicated that the thyristors are preferably provided by SCRs. Bilateral diodes are also connected in parallel across the thyristors.
Huynh et al. U.S. Pat. No. 4,752,866 teaches an ozonator power supply that includes a full wave rectifier for rectifying a three phase voltage, and a full wave bridge invertor using four thyristor switching elements for synthesizing the rectified voltage or DC into an AC waveform for application to the ozonator. A current pulse amplitude control circuit 43 for controlling the conduction of the pass transistor used to control the amplitude of the current pulses. A pulse width control logic and drive circuit 45 are used for controlling the operation of the thyristor switches T.sub.1 through T.sub.4 in a manner providing pulse width control.
Mickal et al. U.S. Pat. No. 4,779,182 teaches a three phase power supply circuit to supply power to an electrostatic filter. As shown in the figures, a three phase AC voltage is rectified by a full wave rectifier and applied to a full wave thyristor invertor circuit. Transformer coupling is used between the invertor and the electrostatic filter.
Masuda U.S. Pat. No. 4,706,182 teaches a radio frequency (RF) high-voltage power supply including a full wave rectifier 8 for rectifying a single phase AC voltage 7. The rectified voltage is applied across a power supply capacitor 6. The voltage developed across the capacitor 6 is applied between the center tap of the primary winding of a transformer 1 and ground. SCR's S.sub.1 and S.sub.2 are connected with their main current paths in opposite polarity relative to one another to the upper and lower connections to the primary winding, respectively. Inductors L.sub.1 and L.sub.2 provide the coupling between the SCR's and the primary winding, respectively. Inverse current diodes are connected in parallel with the main current paths of each one of the SCR's. Conduction of the SCR's S.sub.1 and S.sub.2 are controlled for providing an RF output voltage across the secondary winding 2 of the transformer 1.
Petitimbert U.S. Pat. No. 4,833,583 shows a power supply for processing three phase AC to provide voltage to an ozonizer. For each phase of the three phase AC, a pair of SCR's are connected in parallel with a respective transformer winding.
Von Bargen et al. U.S. Pat. No. 4,048,668 teaches a power supply for driving a high voltage ozonator. The power supply full wave rectifies a single phase AC voltage, which is then chopped via invertor circuits into a high frequency synthesized AC output voltage for driving an ozonator.
Lowther U.S. Pat. No. 4,027,169 teaches a high voltage power supply for driving a corona generator. The power supply includes a pair of SCR's connected in series with the primary winding of a main transformer 38. The gate electrodes of the SCR's are connected across primary windings of a controlled transformer 37.